Introduce theme negotiator for mail and use it in mail manager

Changes are made on on 11.x (our main development branch) first, and are then back ported as needed according to our policies.

Can the deprecations be updated for 11.2

Not sure who to ping to review this one.

Deprecation message updated. Good candidates for reviewers are the maintainers of the mailsystem → module I guess.

The Needs Review Queue Bot → tested this issue. It fails the Drupal core commit checks. Therefore, this issue status is now "Needs work".

This does not mean that the patch necessarily needs to be re-rolled or the MR rebased. Read the Issue Summary, the issue tags and the latest discussion here to determine what needs to be done.

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> Good candidates for reviewers are the maintainers of the mailsystem module I guess.

the maintainer of that module is following this issue, I'll try to review soon.

Fixed issue reported in #13.

Apologize for letting this one sit.

But does need a rebase again

If you are another contributor eager to jump in, please allow the previous poster at least 48 hours to respond to feedback first, so they have the opportunity to finish what they started!

Conflicts due to 📌 Use ::class instead of string literals in plugin managers Active resolved.

Does this require a CR you think?

I guess yes.

The Needs Review Queue Bot → tested this issue. It fails the Drupal core commit checks. Therefore, this issue status is now "Needs work".

This does not mean that the patch necessarily needs to be re-rolled or the MR rebased. Read the Issue Summary, the issue tags and the latest discussion here to determine what needs to be done.

Consult the Drupal Contributor Guide → to find step-by-step guides for working with issues.

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